Knowing the DX6i

Hello There, I own a Spektrum DX6i and I would like to ask your advice on a couple of things, please? I use the radio on my dirt car because I find the stick controls better for me. I am in a wheelchair and I have limited use of my hands and fingers, particularly with my right hand.
I use the the rudder to steer the car (left side of the radio) and elevator for my throttle on the right.

I would like to know if (how?) I can tune the radio to make the rudder go more to the left side than the right, but still steer straight?

I was also wanting to know if there is any advantage in me running the throttle from it’s original position? That is, to the left side of the radio (up and down). The problem I want to try and get rid of is that engine revs (ie: throttle) comes on too fast and the car wants to spin out. Also, if I use this throttle, it has no spring. How may I overcome this?
Thanks a lot for your advice. Steve